feat(ui): add SelectMenu component and replace native selects

Introduce a reusable SelectMenu component and replace native select
controls across the UI. This centralizes select behavior and styling,
enabling grouped options, placeholders, and a consistent popup
interaction. Add comprehensive CSS and light-theme tweaks, and update
rebase action styling to integrate the new control.

- Add a unified SelectMenu component and wire change handlers.
- Implement .select-menu styles, popup behavior, and theme overrides.
- Replace ad-hoc native selects in dialogs and rebase UI for consistency.

src/lib/components/InteractiveRebaseDialog.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { AlertTriangle, ArrowDown, ArrowUp, GitBranch, LoaderCircle, Play, X } from "@lucide/svelte";
   import type { GitBranch as GitBranchInfo, RebaseAction, RebaseCommit, RebasePlanItem } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   interface PlanRow extends RebaseCommit {
     action: RebaseAction;
@@ -41,6 +42,7 @@
   ));
   let invalidReword = $derived(rows.some((row) => row.action === "reword" && !row.message.trim()));
   let canStart = $derived(Boolean(base) && rows.length > 0 && keptCount > 0 && !invalidSquash && !invalidReword && !isLoading && !isBusy);
+  const rebaseActions: RebaseAction[] = ["pick", "reword", "squash", "fixup", "drop"];
 
   function updateAction(index: number, action: RebaseAction) {
     rows = rows.map((row, rowIndex) => rowIndex === index ? { ...row, action } : row);
@@ -82,12 +84,7 @@
       <section class="rebase-base-bar">
         <label>
           <span>Rebase <strong>{currentBranch || "current branch"}</strong> onto</span>
-          <select value={base} onchange={(event) => onBaseChange((event.target as HTMLSelectElement).value)} disabled={isBusy || isLoading}>
-            <option value="" disabled>Select a base branch</option>
-            {#each availableBases as branch (branch.name)}
-              <option value={branch.name}>{branch.remote ? "Remote · " : "Local · "}{branch.name}</option>
-            {/each}
-          </select>
+          <SelectMenu value={base} options={availableBases.map((branch) => ({ value: branch.name, label: `${branch.remote ? "Remote - " : "Local - "}${branch.name}` }))} placeholder="Select a base branch" disabled={isBusy || isLoading} onChange={onBaseChange} />
         </label>
         <p>Oldest commit first. Reorder commits, then choose how each one should be replayed.</p>
       </section>
@@ -110,9 +107,7 @@
                 <button type="button" onclick={() => move(index, -1)} disabled={isBusy || index === 0} title="Move up"><ArrowUp size={14} aria-hidden="true" /></button>
                 <button type="button" onclick={() => move(index, 1)} disabled={isBusy || index === rows.length - 1} title="Move down"><ArrowDown size={14} aria-hidden="true" /></button>
               </div>
-              <select class={`rebase-action ${row.action}`} value={row.action} onchange={(event) => updateAction(index, (event.target as HTMLSelectElement).value as RebaseAction)} disabled={isBusy} aria-label={`Action for ${row.short_hash}`}>
-                <option value="pick">pick</option><option value="reword">reword</option><option value="squash">squash</option><option value="fixup">fixup</option><option value="drop">drop</option>
-              </select>
+              <SelectMenu class={`rebase-action ${row.action}`} value={row.action} options={rebaseActions.map((action) => ({ value: action, label: action }))} disabled={isBusy} ariaLabel={`Action for ${row.short_hash}`} onChange={(value) => updateAction(index, value as RebaseAction)} />
               <code>{row.short_hash}</code>
               <div class="rebase-commit-copy">
                 {#if row.action === "reword"}